This invention relates to a solder connection formed of a tin-bismuth solder alloy and, more particularly, to such solder alloy comprising a tertiary metal, preferably gold or silver, to enhance high temperature properties. In one aspect, this invention relates to a process for forming a solder connection by reflowing tin-bismuth solder paste, wherein the tertiary metal is deposited onto the faying surface and dissolves into the solder liquid during reflow to improve high temperature properties of the product connection.
In the manufacture of a microelectronic package, it is common practice to attach a component onto a printed circuit board or the like by surface mounting utilizing a solder connection formed from a solder paste. For this purpose, the board features a circuit trace that includes a solder-wettable bond pad that constitutes a first faying surface for the connection. Similarly, the component includes a second, solder-wettable faying surface, for example, a contact. The solder paste comprises particles of solder alloy dispersed in a vaporizable vehicle and containing an expendable organic binder. The paste is applied to the faying surface, after which the component is assembled with the second faying surface in contact with the deposit. The assembly is then heated to melt and reflow the solder alloy. Upon cooling, the solder alloy resolidifies and bonds to the faying surfaces to complete the connection. The solder connection not only physically attaches the component to the board, but also electrically connects the trace on the board and the contact of the component to conduct electrical current to or from the component for processing.
The circuit trace in the board is commonly formed of copper. In order to protect the copper surface from oxidation and thereby enhance wetting by the solder alloy, which wetting is essential to forming a strong solder bond, it is common practice to electroplate solder alloy onto the bond pad prior to applying the solder paste. During reflow, solder liquid from the plate and the paste interfuse to form a uniform connection.
Common solder is composed of tin-lead alloy. It has been proposed to form a lead-free solder connection composed of tin-bismuth alloy. However, tin-bismuth alloy in the form that is electroplated or is commercially available in paste exhibits poor mechanical properties at elevated temperatures of the type typically encountered by microelectronic packages during use. In particular, such solder alloys tend to become unacceptably soft at temperatures as low as 100.degree. C. and exhibit a relatively low melting temperature, particularly in comparison to the common tin-lead solders.